What’s the best way to compare lighting efficiency?
“From a human perspective since the early 20th century there was a number of researchers who studied how sensitive the human eye is to the light wavelengths,” said Eden Dubuc, technology leader at Current, powered by GE. “It was discovered that there are receptors in the eyes that collect blue, red and green light. The human eye is most sensitive to yellowish-green light, around 550 nanometers (nm). The reason that humans are really sensitive to this light is because they need to identify good plants as a source of food.
“This color sensitivity of humans varies depending on gender, age and location on the Earth. In order to standardize light measurement, scientists developed a sensitivity curve called a photopic curve that is based on statistical distribution from the world population.”
Dubuc said that it has only been since the 1970s that research has determined plant sensitivity to color. Plants absorb mostly light energy in the range from 400-700 nm, which is defined as photosynthetically active radiation (PAR).
“If white light, which is broad spectrum, is directed at plants, they appear green,” he said. “Plants absorb most of the red and blue light to activate photosynthesis. If a grower wants to create biomass, then blue and red light are the most sensitive to the plants to convert energy in the plants. Even though the ultraviolet (less than 400 nm) and far red (greater than 700 nm) are less useful to the plants for creating biomass, there are some benefits to other aspects of the plants.”
Determining lighting efficiency
Dubuc said lighting efficiency is related to how much electricity goes into the lighting fixture or luminaire and the amount of energy that comes out.
“If 1 watt of electricity is put into a luminaire and 0.5 watt of light comes out, then there is 0.5 watt of heat generated by the luminaire,” he said. “In some luminaires the energy can also be mechanical energy if there are moving parts. If a luminaire doesn’t use active cooling, there aren’t other ways of losing energy. Active cooling is not used in GE luminaires so the watts energy release is divided by two, a portion goes into light and a portion goes into heat.”
Dubuc said the same thing happens to the electricity regardless of whether a grower is using a light emitting diode (LED) or high pressure sodium (HPS) luminaire.
“In a LED the light is mostly in the visible range,” he said. “With HPS there can be a lot of infrared radiation so there is heat radiation from the luminaire that has to be taken into account. Subtracting the amount of watts going into a luminaire from the amount of visible light coming out, everything else is heat, either radiation, convection or conduction.”
In regards to the lighting efficiency of luminaires, Dubuc said most growers are now talking about photon efficacy, which is micromoles per joule. “Micromoles are the quantity of photons,” he said. “Micromoles per second divided by watts creates micromoles per joule, which is photon efficacy. This is an important value when growers are looking at luminaires. This is how growers can compare the efficiency of the luminaire including the ballast and reflector.
“If lighting manufacturers are talking about micromoles per joule, they are measuring the lighting efficiency the same way. If they are using the units, micromoles per joule, it is well defined.”
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